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PHYLUM CHORD AT A 



559 



the effects of shocks. As in the case of the Bird, the folds 

 giving rise to the amnion and serous membrane, may consist 

 from the first (except the head-fold which, being formed from 

 the proamnion, consists solely of ectoderm) of somatic mesoderm 

 as well as ectoderm ; or mesoderm may extend into them later, 

 so that, either from the first, or as a result of outgrowth which 

 takes place subsequently, the serous membrane may contain 

 mesoderm as well as ectoderm, but in some Mammals it appears 



CLF 



TA 



AX 



OL 



YS 



AN 



FIG. 1150. A Rabbit embryo and blastodermic vesicle at the end of the tenth day. The 

 embryo is represented in surface view from the right side, the course of the alimentary 

 canal being indicated by the broad dotted line ; the blastodermic vesicle is shown in median 

 longitudinal section. The greater part of the tail has been removed. AN', pro-amnion ; 

 AX. cavity of amnion ; C. extra-embryonic portion of coelome ; E. ectoderm ; E'. thickened 

 ectoderm by which the vesicle is attached to the uterus and from which the foetal part 

 of the placental is derived ; EGT. ectodermal villi ; El. auditory vesicle ; GF. fore-gut ; GH. 

 hind-gut ; GT. mid -gut ; H. endoderm ; OL. lens of eye ; R. heart ; SI. sinus-terminalis ; 

 TA. allantoic cavity ; YS. yolk-sac. (From Marshall, in part after Van Beneden and Julin.) 



to remain entirely ectodermal. The ectodermal cells trophoblast 

 cells of the serous membrane enter into close relationship with 

 the mucous membrane of the wall of the uterus, and send out 

 processes or villi, by means of which the ovum becomes intimately 

 attached, and by means of which nourishment is absorbed. 



A study of the development of the amnion in various Mammals 

 appears to render it probable that the mode of formation, above 

 sketched, observable in the Sauropsida and some Mammals, is by 



